Can't change my appled id on my iphone after I changed my id on my pc

What happens when you try? Error message(s)? Have you gone to Settings/Your Name, scrolled to the bottom of the page, and tapped Sign out?

See the Apple Support article(s) below.

What to do after you change your Apple ID or password - Apple Support.      

Do you have access to another WiFi network where you can try? That has worked for several people.

Try a restart.

Restart your iPhone, iPad, or iPod touch         

Try a forced restart.

Reset Device.      

If necessary

Hard reset.        

Do a backup and try a factory reset. Then restore from your backup.

Restore to Factory setting.      

Backup - Restore your device from an iCloud or iTunes backup.      

If you log into the old ID, can you then sign out of the old ID?